1. Field of the Invention
The present invention relates to an automotive anti-lock control system.
2. Discussion of Background Art
Conventionally, as automotive anti-lock control systems, there is known in general an anti-lock control system comprising a normally-opened electromagnetic valve provided along a connection line which connects a master cylinder for generating a brake fluid pressure through the operation of a brake pedal and a wheel brake and a normally-closed electromagnetic valve along a release line which connects a reservoir for reserving brake fluid and the wheel brake, wherein, when a wheel is about to be locked up, the normally-opened electromagnetic valve is put in a closed condition, whereas the normally-closed electromagnetic valve is put in an opened condition so that a brake fluid pressure built up at the wheel brake is released to the reservoir via the release line (a pressure decreasing mode), in addition, when the brake fluid pressure at the wheel brake is attempted to be held, both the normally-opened electromagnetic valve and the normally-closed electromagnetic valve are put in closed conditions so that the wheel brake is cut off from the connection line and the release line (a holding mode), and furthermore, when the risk of wheel lockup has disappeared, the normally-opened electromagnetic valve is put in the opened condition, whereas the normally-closed electromagnetic valve is put in the closed condition so that a brake pressure built up at the master cylinder is transmitted directly to the wheel brake via the connection line (a pressure increasing mode), whereby, as required, the braking force is adjusted and controlled so that the slip rate of the wheel does not exceed largely a predetermined value through automatic control of so-called pumping operations by selecting the modes in an alternative fashion.
Then, in this type of automotive anti-lock control system, during the so-called holding mode where the brake pressure at the wheel brake is held as it is, in order to keep the normally-opened electromagnetic valve in the closed condition, a current value corresponding to 100% of the rated current value is applied continuously (for example, refer to Patent Literature No. 1).
In automotive anti-lock control systems of this type, there exists an anti-lock control system which adopts a so-called moderate pressure increasing mode where once an anti-lock control is initiated, a limited pressure increase is repeated every predetermined time in a pressure increasing mode so as to increase macroscopically the brake fluid pressure at the wheel cylinder relatively slowly.
As the anti-lock control system which realizes the moderate pressure increasing mode in a concrete manner, there exists, for example, an anti-lock control system which increases the wheel cylinder pressure in a stepped fashion by energizing the normally-opened electromagnetic valve so as to bring it into a closed condition while canceling the energization to the normally-opened electromagnetic valve every predetermined time so as to form a pulse shape so that the normally-opened electromagnetic valve repeats an opened condition and a closed condition at slightly short time intervals. Then, there exists an anti-lock control system which controls, as this occurs, in order to open the normally-opened electromagnetic valve at a predetermined opening, such that a constant current value of a predetermined duty ratio (DLEV) which is smaller than 100 percent but is larger than 0 percent is applied continuously to the normally-opened electromagnetic valve (refer to Patent Literature No. 1, for example).
JP-A-9-104335 (page 14, FIG. 15A)    [Patent Literature No. 1]
In the aforesaid conventional automotive anti-lock control systems, while the current value of the intermediate duty ratio between 100 to 0 percent is continuously applied to the normally-opened electromagnetic valve in order for the normally-opened electromagnetic valve to have a certain opening in the moderate pressure increasing mode, in this case, a non-linear change in thrust relative to stroke is raised as one of the characteristics of the normally-opened electromagnetic valve. With the electromagnetic valve having such a characteristic, even if the constant current value of the intermediate duty ratio is conducted to the normally-opened electromagnetic valve, a variation in thrust is large, and in the event that there is occurring on the master cylinder side a variation in brake fluid pressure which is attributed to the output pulsation of the pump, the normally-opened electromagnetic valve is largely subjected to effects imposed by the variation so occurring. Due to this, the opening is not stabilized, leading to a problem that the pressure increasing rate in the pressure increasing mode cannot be stabilized.